A HOST-SPECIFIC, NITROGEN-FIXATION MUTANT OF BRADYRHIZOBIUM - PHYSIOLOGY ON 3 HOST PLANTS

NC92 #284 is a transposon mutant of Bradyrhizobium sp. (Arachis) strai n NC92 and has a host-specific fixation phenotype. It appears to be in effective on the host pigeonpea (90% reduction in shoot N compared to that of the wild type), but partially effective on two other host plan ts, groundnut and siratro (50% and 20% reduction in shoot N compared t o the wild type, respectively). To understand this phenomenon of host- specific fixation, the physiological basis of the phenotypes was inves tigated. Host-dependent differences in symbiotic effectiveness were la rgely explained by the degree to which nitrogenase activity was impair ed in the various #284 symbioses. Nodulation and the onset of nitrogen fixation were found to be delayed on all three hosts, but to the grea test degree on pigeonpea. The specific activity of nitrogenase per gra m nodule was also reduced on all three hosts, again to the greatest ex tent on pigeonpea. By contrast. the carbon costs and relative efficien cies of each symbiosis were similar to the wild type. The results indi cate that the host-specific fixation phenotype of #284 is reflected in a quantitative reduction in the amount of N, fixed. Thus the phenotyp es reflect the different ability of the three host plants to tolerate or support the #284 mutation, rather than a defect in a specific inter action between #284 and a particular host plant.